I'll add that it is normal for the male and female to fight like cat and dogs for a few weeks before they pair up (if they pair up). This will likely happen when the female is mature and looking for a mate.All three are sick.
The one with the hard symptoms is beyond saving. I'd euthanize.
The other's should be put in a hospital tank on their own. I hope you haven't introduced something too nasty.
I'm pretty sure you are dealing with some kind of intestinal parasite and at least a bacterial secondary infection.
When you moved the fish to a hospital tank observe them closely. Check the feces, check the behaviour. Do a lot of waterchanges and add humic substances in the form of leaf litter and alder cones.
And for the future QUARANTINE!
Just an addition: Pelvicachormis are no candidates for trios or harems, as the females are not only the ones who chose the partner, they do most of the courtship display and the territorial fighting aswell.
No. As long as it's unclear what they have, any medication might make things worse.Should I try a medication of some sort?